The present invention relates to a method for evaporation of a solution using a recompression evaporation plant which consists of one or more evaporation units and possibly also apparatus for distillation of said solution or other liquid.
The object of the present invention is to raise the dry solid content attained in recompression evaporation of a solution considerably higher than has been practically possible heretofore and to do so without on that account increasing the energy requirement of said recompression evaporation.
The concentration of solutions by evaporation is carried out extensively within various industries. In e.g. the chemical woodpulp industry large quantities of waste liquor from the digestion process are evaporated. The evaporated liquor is then burned in a furnace where steam is produced and from which the inorganic chemicals present in the liquor are recovered for the production of fresh digesting chemicals. At the present time the so-called multi-stage evaporator is the most commonly used type of evaporation plant.
During recent years the price of fuel has risen very rapidly and significantly more rapidly than the price of electricity. As a consequence the other principal type of evaporation plant, viz. the recompression evaporator, has in many cases become significantly more economic than multi-stage evaporation.
In both said principal types of evaporation plant the heat required for vaporization of the solvent (generally water) is transferred to the solution by means of a heating surface. The solution to be evaporated flows on one side of said heating surface and a vapour with a condensation temperature which is higher than the temperature of the solution is condensed on the other side of said heating surface.
The viscosity of a solution generally increases considerably when the solution is concentrated by evaporation. When the viscosity of the solution increases the heat transfer is reduced so that a larger heating surface and/or a larger temperature difference between the condensed vapour and the solution is required.
Moreover, when a solution is evaporated to a high concentration there is often precipitation of inorganic and/or organic material which to a considerable extent fastens to the heating surface in the form of incrustations thereby causing a further deterioration in the heat transfer and making it necessary to clean the evaporation unit concerned at regular intervals.
The elevation of the boiling point of a solution, i.e. the difference between the temperature of the boiling solution and the condensation temperature of the vapour produced, also increases with increasing concentration.
As a consequence of these physical phenomena both the capital costs and the running costs calculated per quantity of solvent evaporated increase rapidly the more concentrated the solution to be evaporated becomes. Because of this the economic and practical limit in the evaporation of e.g. black liquor is at present a final concentration of approx. 65% dry solids content, although from the point of view of combustion and heat economy it would be desirable to carry the evaporation considerably further. The evaporation of black liquor above approx. 60% dry solids content normally also requires so-called forced-circulation evaporation units which are more expensive in terms of both capital and running costs than e.g. Kestner units which can be used for evaporation up to approx. 60% dry solids content.
In U.S. Pat. No. 3,475,281 a recompression evaporation plant for a solution is described in which the liquid is evaporated in two stages of which the first stage is equipped with a recompression unit in order to compress mechanically the vapours from the first evaporation stage and return said vapours with an excess of heat as an indirect heating medium for the evaporation of further solution in said first stage, the surplus vapour from the first stage being used for the evaporation of solution in the second stage.
In a recompression evaporation plant, however, the compressed vapour becomes considerably overheated. It is well-known that with the condensation of an overheated vapour the transfer of heat is slower than with a saturated vapour if the degree of overheating is appreciable. Accordingly in recompression evaporation plants the overheating of the vapour is usually removed by wetting the vapour with condensate before it is led to the evaporation unit for condensation.
The present invention aims to reduce the difficulties which arise as a result of high viscosity, elevation of boiling point and formation of incrustations in the evaporation to high concentration of solutions such as e.g. black liquor.